1. Field of the Invention
The present invention relates to a frequency converter used for a television tuner or the like.
2. Description of the Related Art
FIG. 7 shows a conventional oscillation circuit used for a VHF television tuner, for example, and the oscillation circuit includes a first oscillation transistor 2 and a second oscillation transistor 3 provided within an integrated circuit 20 and a resonating circuit 4 provided outside the integrated circuit 20. The first and second oscillation; transistors 2 and 3 are differentially connected to each other.
The resonating circuit 4 provided outside the integrated circuit 20 is coupled to a base of the first oscillation transistor 2 and a collector of the second oscillation transistor 3 through a terminal 20a. The resonating circuit 4 includes an inductive element 4a and a varactor diode 4b, and one end of the resonating circuit 4 is connected to the terminal 20a and the other end of the resonating circuit is grounded outside the integrated circuit 20. In addition, an oscillation frequency is set by a tuning voltage Tu applied to the varactor diode 4b. In the oscillation circuit, although not shown, a bias voltage supplied from a power supply is applied to bases of the oscillation transistors 2 and 3. A value of the bias voltage is normally constant (for example, refer to JP-A-2004-172934). An oscillation signal is output from the collector of the oscillation transistor 3.
Further, FIG. 8 shows a conventional mixer to which an oscillation signal is supplied. The mixer is also provided within an integrated circuit. A first transistor 31 and a second transistor 32 are differentially connected to each other. A television signal is input to an input terminal 34 connected to a base of the first transistor 31. A base of the second transistor 32 is grounded in a high-frequency manner. A constant bias voltage is applied to the bases of the first and second transistors 31 and 32.
A collector of the first transistor 31 is connected to an emitter of a third transistor 36 and an emitter of a fourth transistor 37. In addition, a collector of the second transistor 32 is connected to an emitter of a fifth transistor 38 and an emitter of a sixth transistor 39. Thus, the third transistor 36 and the fourth transistor 37 are differentially connected to each other, and the fifth transistor 38 and the sixth transistor 39 are differentially connected to each other.
A base of the third transistor 36 and a base of the sixth transistor 39 are connected to each other and are then connected to an input terminal 40 of a local oscillation signal, and a base of the fourth transistor 37 and a base of the fifth transistor 38 are connected to each other and are then connected to an input terminal 41 of a local oscillation signal. The local oscillation signal is balanced-input between the input terminals 40 and 41.
In addition, a collector of the third transistor 36 and a collector of the fifth transistor 38 are connected to each other, and a collector of the fourth transistor 37 and a collector of the sixth transistor 39 are connected to each other. At this time, connection points between the collector of the third transistor 36 and the collector of the fifth transistor 38 and between the collector of the fourth transistor 37 and the collector of the sixth transistor 39 become output terminals 42 and 43 from which intermediate frequency signals are balanced output, respectively. A power supply voltage is applied to each of the connection points through feed resistors 44 and 45 (for example, refer to JP-A-2004-187101).
However, in the oscillation circuit described above, if the capacitance value of the varactor diode 4b is increased by lowering a control voltage in order to reduce the oscillation frequency, a resistance component of the varactor diode is increased and a Q-factor of the resonating circuit 4 is decreased. As a result, the loop gain of the oscillation circuit becomes insufficient, which may reduce the oscillation power, and in an extreme case, the oscillation may stop. In addition, since the loop gain is decreased due to a rise in temperature, there is a possibility that the oscillation power will be reduced or the oscillation will stop. If the oscillation power is decreased, the conversion efficiency of the mixer is reduced, which lowers the level of an intermediate frequency signal that is output. In addition, the gain of the mixer is also decreased due to the rise in temperature.